The present invention relates to polarizers, to polarizing couplers and to polarization beamsplitting couplers for fiber optic applications. It also relates to fiber optic instrumentation, such as single channel fiber optic ring resonators, which utilize a polarized light signal.
Prior art fiber couplers employing lapped coupling blocks have involved constructions utilizing an index-matching material, such as an oil or a bonding agent, between the blocks, or utilizing optical contact bonding of the lapped regions of the fibers.
These constructions all suffer from varying degrees of instability and environmental sensitivity, usually experienced as a coupling ratio shift of the coupler. The refractive index of the coupler index-matching oil, and the fixtures generally employed for maintaining the coupler/core alignment are highly temperature sensitive. The index oil has tendency to dry out, is susceptible to solvent or water absorption and may adversely affect the adhesive which secures the fiber in the coupling block. Index-matched UV curable adhesives allow coupling ratio adjustment on initial set-up, and they cure on demand, but they also have high temperature coefficients of refractive index, although lower than those of index matching oils. Environmental issues such as lifetime, aging parameters and susceptibility to water absorption are unknown for this type of bonding. The alternative approach of optical contact bonding places extreme demands on the coupler polishing flatness and on assembly techniques. In addition, the optical contact bonding process is usually irreversible, making it impossible to effect later compensating adjustments for the inevitable splitting ratio drift after assembly and aging. Optical contact bonded couplers are also sensitive to thermal gradients, and cannot be used for fabricating polarization beamsplitting couplers or for joining dissimilar materials.
Polarization beamsplitting couplers have been fabricated by applying Aluminum thin films having a complex index of refraction of n=1.20-7.0i at 633 nm to the lapped coupling region of a fiber in a coupling block, and joining it to an opposed coupling block using a suitable index-matching oil. A dielectric or second metal layer is sometimes deposited over the Aluminum film to enhance the plasmon effect responsible for polarization beamsplitting. Using these techniques, over 30 dB isolation with 1 dB excess loss have been obtained, and it is possible to obtain complete transmission of one polarization while completely rejecting the other polarization. Polarizers have also been demonstrated using similar techniques involving depositing thin Aluminum or other films on the exposed fiber coupling region of a single coupler half. These constructions suffer from instability due to environmental exposure. The polarization beamsplitting couplers using similar metal film technology have only been demonstrated using matched index oils or possibly matched index bonding agents, constructions which both result in marked dependence on environmental conditions, and in instability. Thin-film polarizers have an exposed thin film of a metal such as Aluminum, which can oxidize if not sealed.
It is therefore desirable to provide a simple, rugged and effective construction for fabricating polarizing elements.